1. Field of the Invention
The present invention relates to a mirror rocking member for use in an optical deflector, in particular, which comprises a movable mirror having a reflective surface and rockably supported, and which rocks the movable mirror to change a direction of the reflective surface and thereby changes the direction of a light beam reflected by the reflective surface.
2. Description of the Related Art
A miniaturized optical deflector, which is constituted by using a micro-machining technique to integrally form a mirror, elastic member, and driving means, has been proposed. In general, for a high-speed operation of the optical deflector, a driving system with a driving frequency compatible with a resonant frequency is used. However, the resonant frequency is inversely proportional to the square root of a moment of inertia of a movable portion with the mirror formed thereon, and is proportional to the square root of rigidity of the elastic member supporting the movable portion. On the other hand, a deflection angle is proportional to a driving force, and inversely proportional to the rigidity of the elastic member. This shows that the moment of inertia of a movable plate is effectively reduced in order to obtain a large deflection angle with a small force (raise a driving efficiency).
Jpn. Pat. Appln. KOKAI Publication No. 10-62709 discloses a galvano-mirror apparatus, which includes an elliptical movable mirror. A light beam for general use has a circular sectional shape, such that a spot projected on the movable mirror will be circular or elliptical. The galvano-mirror apparatus has a movable mirror formed in an elliptical shape, not a rectangular shape, so that the moment of inertia is reduced and the driving efficiency is enhanced. The elliptical movable mirror has a moment of inertia reduced to about 60% as compared with the rectangular movable mirror.
In the galvano-mirror apparatus, with reduction of the moment of inertia of the movable mirror, an area of a driving electrode for rocking the movable mirror is also reduced. When a whole back surface of the movable mirror is assumed to be a driving electrode, a driving torque of a neutral time by an elliptical electrode is about 66% of the driving torque of the neutral time by a rectangular electrode. Since a decrease ratio of the driving torque of the neutral time is smaller than that of the moment of inertia, the driving efficiency is generally improved, but this cannot be said to be a remarkable improvement.
Moreover, the moment of inertia of the movable mirror is reduced by decreasing a thickness of the mirror without changing the shape of the mirror. A digital micro-mirror device (DMD) prepared by the micro-machining technique has mirrors, each of which has a size of an about ten micrometer square, and which are integrated in an array. The mirror is formed of an Al thin film, has a remarkably small moment of inertia, and can therefore operate at a high speed.
The reduction of the thickness of the movable mirror deteriorates the rigidity of the movable mirror. In the DMD, since the mirror is as small as about 15 μm square, a large problem does not arise. In a mirror having a size of several millimeter squares, however, excessive reduction of the thickness of the mirror deteriorates a dynamic flatness.